python-tornado tests fail against python3.5

The python-tornado package's autopkgtests fail now that python3.5 is enabled. Strangely this failure is not caught at package build time, only at autopkgtest run time.

=== python3.5 ===
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/tornado/test/runtests.py", line 180, in <module>
    main()
  File "/usr/lib/python3/dist-packages/tornado/test/runtests.py", line 169, in main
    tornado.testing.main(**kwargs)
  File "/usr/lib/python3/dist-packages/tornado/testing.py", line 692, in main
    unittest.main(defaultTest="all", argv=argv, **kwargs)
  File "/usr/lib/python3.5/unittest/main.py", line 93, in __init__
    self.parseArgs(argv)
  File "/usr/lib/python3.5/unittest/main.py", line 140, in parseArgs
    self.createTests()
  File "/usr/lib/python3.5/unittest/main.py", line 147, in createTests
    self.module)
  File "/usr/lib/python3.5/unittest/loader.py", line 219, in loadTestsFromNames
    suites = [self.loadTestsFromName(name, module) for name in names]
  File "/usr/lib/python3.5/unittest/loader.py", line 219, in <listcomp>
    suites = [self.loadTestsFromName(name, module) for name in names]
  File "/usr/lib/python3.5/unittest/loader.py", line 204, in loadTestsFromName
    test = obj()
  File "/usr/lib/python3/dist-packages/tornado/test/runtests.py", line 60, in all
    return unittest.defaultTestLoader.loadTestsFromNames(TEST_MODULES)
  File "/usr/lib/python3.5/unittest/loader.py", line 219, in loadTestsFromNames
    suites = [self.loadTestsFromName(name, module) for name in names]
  File "/usr/lib/python3.5/unittest/loader.py", line 219, in <listcomp>
    suites = [self.loadTestsFromName(name, module) for name in names]
  File "/usr/lib/python3.5/unittest/loader.py", line 153, in loadTestsFromName
    module = __import__(module_name)
  File "/usr/lib/python3/dist-packages/tornado/test/auth_test.py", line 8, in <module>
    from tornado.auth import OpenIdMixin, OAuthMixin, OAuth2Mixin, TwitterMixin, AuthError
  File "/usr/lib/python3/dist-packages/tornado/auth.py", line 145, in <module>
    class OpenIdMixin(object):
  File "/usr/lib/python3/dist-packages/tornado/auth.py", line 155, in OpenIdMixin
    callback=None):
  File "/usr/lib/python3/dist-packages/tornado/concurrent.py", line 428, in return_future
    replacer = ArgReplacer(f, 'callback')
  File "/usr/lib/python3/dist-packages/tornado/util.py", line 287, in __init__
    self.arg_pos = inspect.getargspec(func).args.index(self.name)
  File "/usr/lib/python3.5/inspect.py", line 1038, in getargspec
    stacklevel=2)
DeprecationWarning: inspect.getargspec() is deprecated, use inspect.signature() instead

See https://jenkins.qa.ubuntu.com/job/wily-adt-python-tornado/12/ for full details.